Pneumothorax is the presence of gas (most usually air) in the pleural space. Pneumothoraces can be primary, when no apparent lung disease is detected in the background (there are caveats, however). Pneumothoraces may be secondary (occurring in the presence of a background lung disease that predisposes to disruption of visceral pleural integrity); they may be spontaneous or traumatic, open or closed, small or large. Each of these attributes has clinical relevance that has been explored in this chapter. This chapter discusses the conditions to be sought when confronted with a pneumothorax, and discusses the pathophysiology of the various kinds of pneumothoraces. It explores the caveats associated with the diagnosis of primary pneumothorax, and the clinical implications of each of the usual conditions associated with secondary pneumothoraces. The chapter also considers the implications of the size of a pneumothorax and the algorithms that help in quantifying the amount of gas within the pleura. The relative accuracy of methods used in the detection and quantification of pneumothoraces is also considered. A discussion on pseudopneumothoraces and giant bullae also appears in this chapter.